The geology and evolution of a the Near-Earth binary asteroid system (65803) Didymos.

Abstract Images collected by the DART and LICIAcube spacecraft provide the first resolved views of the Didymos binary asteroid system. These images reveal that the primary asteroid, Didymos, is flattened and has a non-circular equatorial perimeter. At high elevations, its surface is undulating and contains large boulders and craters; at low elevations its surface is smoother and possesses fewer large boulders and craters. Didymos' moon, Dimorphos, possesses a uniform surface covered by boulders, several asteroid-wide lineaments, and a handful of craters. The surfaces of both asteroids include boulders that are large relative to their host body, suggesting that both asteroids are rubble piles. Models run to explain the observations indicate that Didymos has a surface cohesion ≤1 Pa and an interior cohesion of ~$10 Pa, while Dimorphos has a surface cohesion of <0.9 Pa. Crater size-frequency analyses indicate the surface age of Didymos is 40-130x older than Dimorphos, with likely absolute ages of ~12.5 Myr and <0.3 Myr, respectively. Solar radiation could have increased Didymos' spin rate leading to internal deformation and surface mass shedding, which likely created Dimorphos. If part of the Bauptistina family, Didymos likely represents the latest of multiple asteroid generations stemming from the original parent.